Design criteria for an ideal audio-frequency amplifier include maximization of energy efficiency (and thus minimization of heat production), minimization of weight and minimization of bulk.
One type of highly efficient audio-frequency amplifier is known as a “class-D” amplifier. In a class-D amplifier, an input signal controls a pulse generator to produce a sequence of pulses of differing widths. These pulses are averaged over time (such as by using a low-pass filter) to produce an output signal. Because the pulses are of fixed amplitude, the switching elements (which are usually MOSFETs) are switched either completely on or completely off. A MOSFET operates with the lowest resistance when it is fully on and thus has the lowest power dissipation when in that condition, except when fully off where no power is consumed or drawn.
The invention described in U.S. Pat. No. 8,289,097 assigned to Hypex Electronics B.V. (the “Hypex patent”) discloses a pulse width modulation loop of a simple construction which disturbs the input signal to be processed as little as possible. The circuit can be incorporated into audio-frequency amplifiers such as a class-D amplifier.
Specifically, and referring to FIG. 6 of the Hypex patent, the Hypex patent is concerned with a post filter fixed frequency converter with an external sawtooth waveform. This sawtooth waveform is corrected by factors that represent the behaviour of the various parts of the circuit which include the filter, the feedback network, “the forward path” and the comparator. The invention disclosed in the Hypex patent is limited to “fixedly” determined or known clock circuits as described in paragraph [0023] of that document.
The Hypex patent also relies on predetermining the response of the amplifier by extensive internal measurement circuits so that the various factors can be determined. Of course these factors are very complex and are products of temperature, load, and frequency. To accurately determine these factors over these ranges requires significant computational effort.
Further, ‘Improved analogue class-D amplifier with carrier symmetry modulation’ B. H. Candy and S. M. Cox, Audio Engineering Society 117th Convention, 28-31 Oct. 2004 San Francisco, Calif., USA Convention Paper 6260, looks at reducing distortion in a class-D amplifier application. The analogue class-D amplifier disclosed follows the well known prior-art class-D structure with negative feedback, but includes modulation of the symmetry of the carrier oscillator waveform by a derivative of the input signal. The disclosed circuit compensates a nonlinear phase modulation effect that is intrinsic to the prior art structure.
Embodiments of the present invention provide circuits for generating pulse waveforms which are particularly suitable for, but not limited to, controlling the known class-D circuit. Furthermore, embodiments of the present invention aim to ameliorate some of the disadvantages of the prior art.